Geology: Geologic Time

Absolute Dating Methods

1 unstable atom degrades into another.

Fundamental Principles of Relative Dating

1.) Principle of Superposition In an undisturbed succession of sedimentary rock, the oldest layer of rock is at the bottom and the youngest layer is at the top; In undisturbed sedimentary strata, layers of rock increase in age with depth. Credited to Nicolas Steno. To put something on top, something has to be on bottom.

Principles of Relative Dating: Practical Use

3-D model Limestone looks like bricks. The slanted bricks are Dolomite. Sand is dots and Shale is slashes. Silty clay is a mixture of dots and slashes. Layers ABCDEFG are laid down horizontally then they are uptilted through tilting and faulting and an erosional surface is created between them. A fault occurs with the tilt and they are back-to-back; we are uncertain which is first, they are more simultaneous. Afterwards layers JKL are deposited leaving an angular unconformity I. M intrudes and erosional surface N is created. PQS are deposited and then R is injected probably synchronously with S.

Unconformities

A break in the geologic record represented by an erosional surface separating younger strata from older rocks. Gaps - missing portions. Erode - strip away rock when sea level (coastline) changes.

4.) Principle of Cross-cutting Relationships

An igneous intrusion or fault must be younger than the rock it intrudes or cuts across; Credited to James Hutton. Through layers of rock then layers had to exist there first.

Disconformity

An unconformity above and below which the rock layers are parallel; Often very difficult to discern - usually need to look at fossil assemblages. Maybe sea level fluctuates often. Rocks above and below look the same. 5 billion years of missing time.

James Ussher - Ireland (1581-1656)

Based age of Earth on Old Testament genealogy. 6000 years old Earth (Dinosaur Valley State Park) 1701 - an authorized version of the Christian Bible was published that accepted this date in Christian Doctrine. Not challenged for about 100 years - heresy to question. 14th/15th century people tried to age Earth (dates) Used religion funded by church to hush people up, destroyed other proof regularly.

Georges Louis de Buffon - France (1707-1788)

Based age of Earth on cooling/crystallization rate of molten materials (iron balls). Extrapolated measurements to account for the size of the Earth. 75,000 year old Earth (significantly older than 6,000). Condemned by the Catholic Church in France and his books were destroyed. Scientific approach with volcanoes. Earth - molten ball of lava, over time outside cools. Average size of Earth - extrapolated from experiments.

John Joly - Ireland (1857-1933)

Based age of Earth on ocean salinity, assumed that the oceans were originally fresh water (comes from dissolving stuff on land). Compared salinity of river water to sea water. 90 million year old Earth Was believed that continents stationary land masses just eroded, no creation of new material leads to flat Earth. Dissolved material increases as water gets further inland. Start to see processes - product of erosion - using geochemistry.

Lord Kelvin - Ireland (1824-1907)

Claimed that the Principle of Uniformitarianism were invalid and questioned establishment. Based age of Earth on cooling rates and geothermal gradients. 20 to 400 (potentially) million year old Earth. Famous for creating the Kelvin temperature scale. Thick old rock is warmer. Rate and scale of past is NOT the same as present. Cool then places Earth further back in past. Opposite with hot places. Kinda right, lots of variation. Geothermal gradient: cold ground vs. hot spots. Ex: Hurricane tears up beach and dumps sediments in Bay - takes 12 hours, very fast, pulse of intensity over time shows repetitive nature. Some areas are more stable, some are less stable and there is a lot of variation.

Radiometric Dating

Common Methodologies: Radiocarbon Dating: Ratio of 14-C (unstable) to 12-C, 14-C formed by cosmic bombardment, Decays to 14-N electron capture, Half-life = 5730 years, 50 kya limit for age dating, Useful in dating organic materials. (Short-term, Archaeology) Potassium-Argon Dating: Ratio of 40-K to 40-Ar, 40-Ar formed by electron capture, Half-life = 1.3 billion years, Lower limit of 100 kya for age dating, Useful in dating clays. (Origin of Earth = 12% of original material, feldspars, orthoclase, plagioclase) Uranium-Lead Dating: Ratio of 238-U to 206-Pb, 206-Pb multiple alpha and beta decays that occur rapidly, Half-life = 4.47 billion years (very long), 1 million years lower limit of age dating, Useful in extremely old rock. (Decay gives internal heat and fuels plate tectonics) Uranium-Thorium Dating: Ratio of 234-U to 230-Th, 230-Th formed by alpha decay, Half-life = 248 thousand years (much shorter 1/2 life), Lower limit of 350 kya for age dating, Upper limit of 650 kya for age dating, Useful in dating speleotherms. ***Many other radiometric dating methodologies exist

Rock Unit Correlation

Correlation: demonstration of the physical continuity of rock units or biostratigraphic units, or demonstration of time equivalence as in time-stratigraphic correlation. Edge of the flood valley is as far as the sediment can go. Sedimentary rocks are horizontally laid and laterally continuous. Grand Canyon. Water slows down closer to the coast so larger particles settle out. In lower energy, more settles out with the reduced suspension. Go out then more silt and less sand and more clay settles out in the middle of the ocean. Determining what layers of rock are the same. Fossil succession helps with identification of correlation. Windhill Apartments - vertical exposure of rock. Thinning and thickening of rocks tells how the environment changes.

Electron Spin Resonance (ESR)

Dating methodology that measures the concentration of displaced electrons to the average strength of a rock. Elements are bombarded by natural radiation, which can knock a few electrons out of stable minerals. About 1 mya limit. Low accuracy. Destruction of crystal or mineral (rips holes in the crystal lattice). Obliterated with many radioactive striations = OLD rock.

Radiometric Dating (Radioactive Dating)

Decay Rate: the rate at which radioactive isotopes undergo alpha decay, beta decay, or electron capture. Half-Life: Time necessary for half (50%) of the original number of radioactive atoms in an element to decay to a stable daughter product. Ex: Tridium - Start with 100 g H3, 12.5 years later you have 50g, 12.5 more years and you've got 25g, give it another 12.5 years and you're at the good old 12.5g. Shallow groundwater tracing byproduct of Hydrogen bonds.

James Hutton - Scotland (1726-1797)

Father of modern Geology Established the Principle of Uniformitarianism: the same processes acting today operated over the vast majority of geologic time. AKA the present is the key to the past. Looked at water mostly. Concluded what happens now, happens in the past (at intervals). Natural processes. Soft-bodied sea slugs/jellyfish NOT preserved in fossils.

Guide Fossils

Fossils that are easily identifiable, geographically widespread and that existed for only a geologically short period of time. If Coral lived on mud, it would clog the filtration system and cause it to suffocate. Constrained to substrate they live on, dependent on environment they live in. Ex: mud vs. gravel. Marine snail climbing around the bottom = benthic. Swimmers are better tools.

6.) Principle of Fossil Succession

Fossils, and especially assemblages of fossils, succeed one another through time in a regular and predictable order; Credited to William Smith. Biological buildup not horizontal: reefs/mounds. 1.) Life has varied in the past; 2.) Fossil assemblages are recognizably different from one another; 3.) Relative ages of fossil assemblages can be determined. Problems: thought was extinct, but actually still alive. Peliosaur - Lochness monster: evolved two different times, superficially look the same. Coelocanth (lobe-finned fish): thought to be extinct, found some off the coast of Madagascar.

Concurrent Range Zones

Geologic ranges for two or more fossils that overlap with independent first and last occurrence for each fossil type. Amonites - shell and similar to squid in body structure. Extremely common. Global - floating plankton, trolobytes. Not rock units, correlate based on time equivalency. Different answers based on approach = problem with relative data.

Geologic Time Overview

History of Geologic Time Relative Dating Methods (older vs. younger) Stratigraphic Correlation Absolute Dating Methods (dates) The Geologic Time Scale What is time?

Marie Curie - Poland (1867-1934) Pierre Curie - France (1859-1906)

In 1903 they discovered that radioactive decay produces heat. Provided mechanism for Earth's internal heat. Radioactive decay provided a way to date the age of Earth materials. Physicist and chemist, respectively. Revolutionized everything. Uranium gives off heat and energy. Internal heat source keeps Earth melted and churning. Laid groundwork for nuclear weapons (atomic bomb), wanted to harvest energy source - died of cancer.

5.) Principle of Inclusions

Inclusions or fragments in a rock unit must be older than the rock unit itself. Chunks of one type of rock inside another Ex: Conglomerate. Bit of melting rock survive as inclusions (fragments). Intrusive sill vs. Lava flow? Sill is squirted along 2 planes in layers of rock. Lava flow is where there are bits and pieces of basalt present in surface flow or sill injected laterally. Upper contact has bits and pieces then it is definitely a sill. Thermal heating did NOT affect the top then it is a lava flow. Clasts of lava may be present in the overlying younger layer. Strands and shreds of underlying sedimentary material may be in the base of the flow. Baking zone at the bottom of the flow. Rubble zones may be present at the top and the bottom of the flow. [Small-scale, pocket gophers]

Strata Correlation

Key Beds - laterally extensive beds that are sufficiently distinctive enough to allow the identification of the same unit in different areas (Ex: ash layer, coal bed). McKinney Falls in Austin, TX has a layer under the waterfall made up of ash (green clay) deposited in the Cretaceous period through South and Central TX. 10,000 years (short period of time) ago the fossil fauna is NOT that different. Ex: Cycle of limestone and sandstone. Difficult with repetitive succession. Boulder, CO has a green layer that was deposited 66.5 million years ago. Independent of regularly depositional sediments. Ex: atmospheric deposition - major volcanic eruptions (ash). Volcanic ash = cooling globally. Ash layer in the US found on Mt. St. Helena, Washington. Greenland and Iceland (volcanic dust shut down air traffic). Meteor impact in the Cretaceous/Tertiary boundaries - dinosaurs went extinct - found all over Earth with a high concentration of Uridium.

Example of Rock Unit Correlation

Large-scale regional depositional history can be seen with a complete section of time. You can see it across the Grand Canyon, Zion, and Bryce National Parks.

Lithostratigraphy and Biostratigraphy

Lithostratigraphic units: supergroup, group, formation (cyclic pattern), member, bed (thin layer). Biostratigraphy units: Eonotherm (eon), eratherm (era), system (period), series (epoch), stage (age).

Dendrochronology

Method of dating based on the analyses of tree-ring growth patterns. Ages established by cross-dating correlation. 0-14 kya age range. Usually limited to arid regions. Old mudslide with preserved woody material and compare to living trees.

Paleomagnetic Dating

Methodology for measuring the residual orientation of magnetic grains in sediment or igneous rocks that were oriented during deposition or crystallization; Age determined by magnetic orientations of sediment relative to magnetic reversals throughout time. Maximum limit is about 250 mya. Wobble: Patterns of North Pole and Inversions. sequence, pattern, match with known magnetic reversals of Earth. +N+N+S

Charles Lyell - Scotland (1794-1875)

Published "Principles of Geology" (first recognized Geology book) in 1830. Recognized that present day processes could, over long periods of time, have tremendous cumulative effects. No knowledge of plate tectonics, new mountains. Small-scale processes = huge effects. Volcano (magma)

Radioactive vs. Stable Isotopes

Radioactive Isotope: an element capable of changing spontaneously into another element by the emission of charged particles from the nuclei of its atoms. Used for age dating. Gives off alpha and beta particles. Will decay because it's unstable. Stable Isotope: an element that is unchangeable but has an imbalance of protons and neutrons in the nuclei of its atoms. Used for climate reconstruction. Do not degrade. Variation of atom 13-C is stable, but not 12-C. Lighter or heavier one may be preferentially used by plants. Rain at coastline has a higher concentration of 18-O because it's heavier so it falls out faster vs 16-O.

Cosmogenic Radionuclides (CR)

Radioactive elements produced when high-energy cosmic rays (solar radiation) interact with the nucleus of an atom. Quartz impacted by solar radiation at the Earth's surface will develop trace amounts of 26-Al and 10-Be at a ratio of 6 to 1. Radioactive decay begins when mineral is removed from solar radiation. Decay Rate for 26-Al and 10-Be is different. 10s kya to about 5 mya. NOT set by crystallization of material. Beryllium.

Absolute Dating Methods

Radiometric Dating (Radioactive Dating): a technique used to date materials based on a comparison between the abundance of a naturally occurring radioactive isotope and its decay products. Isotope: One of two or more species of the same element, which have the same number of protons in the nucleus, but differing from one another by the number of neutrons. Technology is improving thanks to the Curies, may they rest in peace.

How do we measure it?

Relative Dating and Absolute Dating. How old is the Earth? 4.6 billion years old (determined with radioactive decay) Mesozoic: 250 mya - 65 mya was the time of reptiles/dinosaurs. Paleozoic: 540 - 250 mya. Precambrian had the oldest single-celled animals. 66 mya to today is the age of mammals. Plate tectonics were accepted in the 1960s; destroyed completely, so not sure of 'oldest' rock. Universe = 14 - 16 billion years ago Evolution of animals was a helpful diagnostic tool. Buried deeper => older. Fossils of dinosaur then you know it has to be 60-70 million years ago, but less than 200 million. 10 million years ago if it's a Sabertooth Cat. Canyons are shaped by geomorphic (water) processes sculpting the land surface.

Relative vs. Absolute Dating

Relative Dating: Fundamental Principles of Geology - Superposition, Original Horizontality, Lateral Continuity, Cross-cutting relationships, Inclusions, Fossil Succession. Absolute Dating: Modern Technology - Radiometric Dating, Cosmogenic Radionuclides, Electron Spin Resonance, Paleomagnetism, Dendrochronology, Many more. (Changing a lot less now) NOT uniform in size, established for hundreds of years.

Problems in Methodology

Sample limitations: Low isotope concentrations (trace amounts, not major constituent), Igneous rocks (crystallization rate source unknown), High temperature metamorphic rocks (lots of energy, atom may escape, leakage), Few sedimentary rocks. At 0K the atom would stop spinning. Entropy is the degree of disorder which gets bigger with heat. Sample stability: Closed system required, Leakage can occur if rock is heated or subjected to intense pressure. Cooling doesn't cause problems. Accuracy of equipment - accurate if all in the same area, but not precise if not in the right area (bull's eye). Repeatability necessary (concordant vs. discordant)

3.) Principle of Lateral Continuity

Sedimentary rock layers spread out in all directions until they pinch out or terminate against the edge of a depositional basin. Credited to Nicolas Steno. Coral reef - Atlantic ocean pushes in nutrients. West side - Savanna plane underwater (same thing) Calcareous, limestone mud. <30 ft H2O depth. Bahamas - Bohemian platform = 1/3 of TX. Flood the channel - marine basin dumping sediments. Depends on location - riverbed, beach. There are limits. Stratum terminates by grading into different kind of sediment. Mediterranean - Atlantic Ocean. Sahara dust falls into ocean (central portion away from continents). Stratum terminates by thinning at margin of sedimentary basin. Rock layers extend until they grade into those produced in aqueous environments. [Boundaries]

2.) Principle of Original Horizontality:

Sediments are deposited in horizontal or nearly horizontal layers/planes. Credited to Nicolas Steno. Uptilted rocks assume originally laid horizontally. Flat layer in riverbed after flooding. No plate tectonics explanation so they assumed earthquake and cavern/void opened up in subsurface. Really 2 plates are compressed/pushed together.

Relative Dating

The placing of geologic events in a sequential order as determined by their position in the geologic record. Nicolas Steno - Denmark (1638-1686): "Father of Geology and Stratigraphy" (layering effects of rocks in relation to each other). Fundamental Principles of Relative Dating. All people from Europe - China, Egypt figured it out too. Reinvented the wheel.

Radioactive Decay

The spontaneous disintegration of the atoms of certain nuclides into new nuclides, which may be stable or undergo further decay until a stable nuclide is finally created. Parent nucleus goes to the daughter nucleus after losing a neutron. Alpha Decay: 2 protons and 2 neutrons lost Beta Decay: beta particle (electron) emitted from nucleus Electron Capture: beta particle (electron) captured by nucleus and converted to a neutron.

Stratigraphy

The study of the composition, origin, aerial distribution, and age relationships of sedimentary rocks. (traditional) Lithostratigraphy: rock units defined by their physical characteristics which can be correlated over large distances - sandstone, limestone, shale. Formation, groups, members, and beds. Biostratigraphy: strata defined by the fossils it contains, which may not include rocks of similar composition, biozone. (Age equivalence, snails migrate but may not get there until 10 my later, still more accurate). Chronostratigraphy: correlation of strata strictly based on the absolute age of the rock units. Requires markers that can be specifically dated (trees).

Angular Unconformity

Unconformity below which older rocks dip at a different angle than overlying strata. Principle of original horizontality. Deposited horizontally originally. Central New Mexico. Deposit - erode - re-expose after submerging.

Nonconformity

Unconformity in which stratified sedimentary rocks overlie an erosion surface cut into igneous or metamorphic rocks. Principle of Inclusions. Enchanted Rock is a big pile of Granite dropped with rocks on top. Pluton with weathered surface.